MIGRATION AND DISPERSAL OF INSKCTS : FINAL CONSIDERATIONS. 317 



butterflies and moths, are still very obscure. The migration of those 

 insects whose imagines are voracious to the highest degree, as those of 

 locusts, are evidently based on the absolute necessity of supplying 

 their own immediate wants with food. Dragonfiies are also voracious 

 ia the imago state, and their migrations may probably be due also to 

 necessities of food, or still more probably to the drying-up of their 

 breeding-grounds. The migration of ants also, has no doubt its 

 origin in necessity, and their spread is evidently to prevent over- 

 crowding and for the purpose of cross-fertilisation. Their migratory 

 swarms appear to be largely confined to the male sex, the latter 

 taking voluntarily to Hight, whilst the workers, in many species 

 at least, actually drive oft" the females from the nest in which 

 they have been reared. As the females are driven off at the time 

 that large swarms of males are on flight, and pairing takes place 

 whilst on the wing, close interbreeding is thus prevented, and 

 the physical excellence of the insects is maintained. But what 

 are we to say about butterflies and moths whose only food is the 

 sweet nectar distilled in some fairy cup in the bosom of a charming 

 flower, who leave a flowery Eden in subtropical lands and take vast 

 journeys by sea or land, not to advance their own welfare, but for tbe 

 sake of their yet unborn progeny ? 



Is any explanation of this forthcoming ? It is a generally accepted 

 opinion among scientific men that the orthoptera, to which locusts 

 belong, is one of the most ancient orders of insects. It is further 

 believed that those insects which show the greatest and most marked 

 ditterences, between their various stages (larva, pupa, imago), 

 are the most recently developed, whilst those which have no very 

 marked ditterences in their various stages are the oldest. If this be 

 so, it follows that those which have undergone the greatest amount of 

 difterentiation from the ancestral type, such as hymenoptera, diptera, 

 coleoptera, lepidoptera, will be likely to retain some slight traces of the 

 habits of their ancestors, and if this be allowed, and if, further, the 

 general opinion that the lepidoptera have been developed from offshoots 

 of the neuroptera, and these again, farther back, from forms related to 

 the orthoptera, be true, it becomes evident whence our lepidoptera may 

 have first derived the migratory instinct which some of them still 

 possess in so high a degree. 



There is, of course, no reason why, with the instinctive faculty of 

 migration present, it should not become active when the parent is 

 seeking for food whereon to lay eggs as much as when the parent feels 

 the necessity of obtaining its own food. It must be borne in mind, 

 however, that males as well as females, at least of some species, 

 migrate, and the instinctive faculty that would lead the females to 

 migrate for egg-laying purposes, would hardly find a potent response in 

 the males, unless the faculty were of a more generalised character than 

 the premise suggests. The migrating instinct in insects seems to have a 

 deeper-seated origin than this, and one is inclined to look upon it as a 

 survival of a very old habit which has been transnntted through past 

 ages, and preserved where, for any purpose whatever, its use has been 

 of advantage to its possessor. It is very doubtful how far some of the 

 instances recorded in the preceding chapters are really migratory 

 movements at all. Especial reference is now made to the records 

 relating to the genus CaUulrya.s and allied genera in Central and 



